There is a widespread need for hand-held flashlights and lanterns. One common flashlight includes a two-cell battery for power, an incandescent lamp to emit light, and a simple single-pole switch to connect and disconnect the battery to the lamp. Other flashlights use other numbers of battery cells in order to provide a voltage suitable for various particular conditions. Lanterns often use a fluorescent tube to emit light. Certain keychain fobs use a pair of hearing-aid cells and a red-light light-emitting diode (LED) in order to provide short-range lighting such as might be needed to find a keyhole in the dark.
Battery technology is such that as electrical power is withdrawn from a battery cell, the voltage available across a given current load will decrease. This decreased available voltage across the given load causes reduced light output, gradually dimming the light as the battery charge depletes.
Further, LEDs have voltage, current, and power parameters that must be controlled in order to maximize device life. Commonly, a current-limiting resistor is placed in series with an LED in order that only a portion of the voltage drop from the battery is across the LED and the rest of the voltage drop is across the resistor. This voltage drop and corresponding power loss in the resistor is dissipated as waste heat, which is inefficient for a flashlight which should be designed to emit light.
In addition, it is awkward or difficult to determine the amount of remaining charge in a battery cell, generally requiring removal of the battery from the flashlight in order to measure the remaining charge. In addition, the cost of a separate measurement device can be a negative for this market. Some battery cells today include a built-in liquid-crystal indicator for the charge in the cell, but such a solution requires a separate measurement device/indicator for each battery, and requires removal of the battery from the flashlight in order to perform the measurement and observe the indication of remaining power.